In 1948, a teacher named Bertram Forer gave his students a personality test. A week later, each student got "their own" result. The students said the results were very accurate.

But every student got the same text! Many sentences came from a fortune-telling book. This is called the Barnum effect. When words are vague and kind, we feel they are about us. So be careful when a fortune seems true.

Why do horoscopes and personality quizzes so often seem accurate? In 1948, psychologist Bertram Forer gave 39 students a personality test. A week later, each student received a "personal" description and rated how accurate it was from 0 to 5. The average score was about 4.3.

In fact, every student had received exactly the same description, much of it taken from an astrology book. This tendency to accept general statements as personal is called the Barnum effect. Effective statements often include both sides of a trait, flattery, and vague situations that apply to almost anyone.

Later reviews confirmed that people accept such feedback more easily when it is positive and seems written just for them. Feeling that a reading is accurate does not prove it is.

In 1948, the American psychologist Bertram Forer played a revealing trick on his class. Thirty-nine students completed a personality test and, a week later, each received what appeared to be an individual interpretation. Asked to rate its accuracy on a scale from 0 to 5, they gave an average of about 4.3. Every student, however, had received an identical text, assembled largely from a newsstand astrology book.

The tendency to accept generic descriptions as uniquely personal became known as the Barnum effect, after the showman P. T. Barnum. Forer's demonstration had no control group, but numerous later experiments have produced similar results, and reviews in the 1980s showed that acceptance rises when statements are flattering, vague, double-sided, and presented as tailored to the individual, particularly by a trusted source.

The effect has limits; negative or highly specific statements are accepted less readily. It also bears on what counts as evidence. A high "that's so me" rating measures acceptance, not validity, which is why the felt accuracy of horoscopes, blood-type readings or online quizzes cannot justify them. A 2014 Japanese study of large survey data found that blood type explained less than 0.3 percent of individual differences in personality responses.